|M.Sc Student||Shaltieli Hila|
|Subject||Parametric Investigation of Aluminum, Tungsten and|
|Department||Department of Design and Manufacturing Management||Supervisor||Professor Emeritus Alon Gany|
The objective of the study was to investigate experimentally the ignition characteristics as well as the thermal and energetic performance of aluminum / tungsten / fluor-polymer mixtures. The research contributes significantly to the knowledge regarding these compositions, which can be integrated in a variety of applications where high-energy materials as well as high energy densities are required.
In order to examine the parameters under study, different types of mixtures were prepared and a variety of measurements and comparative experiments were conducted.
Different Fluor-polymers, characterized by a high fluorine content, low melting point, and superior mechanical properties were used as oxidizers. Several types of aluminum powders differing in particle size served as the fuel. Tungsten additive was incorporated for increasing the density of the composition but, as learned, it did not contribute to the caloric output of the composition, lowering the overall specific energy.
TGA (Thermo-Gravimetric Analysis) and DSC (Differential Scanning Calorimetry) are two well-known thermo-chemical tools serving for exploring the thermal behavior of the mixtures. These methods characterize the reactant state at reaction onset; aluminum is in a solid state, flour-polymer is in a liquid state. However, reaction continuation depends on oxidation gas species that appear as polymer degradation begins. It was found that by increasing the weight percentage of tungsten in mixtures containing ETFE polymer, the reaction onset temperature decreased. As tungsten does not react with fluor-polymer, the conceivable explanation is that it improves the mixture’s heat conductivity. Pre-ignition reaction (PIR) was observed in DSC curves of mixtures containing PFA polymer and tungsten additives. It can be attributed to the formation of semi-stable intermediate products.
The caloric output of the composition was measured by PARR 6200 Isoperibol Calorimeter. The highest caloric output was obtained for Al / PFA mixture. In addition, for mixtures containing tungsten powder the results imply that tungsten does not participate in the reaction, decreasing the caloric output in inverse proportion of its content.
Pressure measurements were conducted in a closed vessel. According to the results, there is a direct relationship between aluminum particle size and time for onset of pressure rise and pressure peak. The smaller the aluminum particles, the higher the pressurization rate is. On the contrary, increasing particle size causes a decrease in peak pressure.
With increasing the content of tungsten, the peak pressure reduces, but there is no direct trend for onset and peak pressure times. Another interesting conclusion relates to Teflon, which demonstrated the fastest reaction and highest ignition sensitivity behavior, compared to all flour-polymers. On the contrary, mixtures using the polymer ETFE showed very low ignition sensitivity.
Calculations for the pressure evolving in the vessel were made for aluminum / Teflon mixture and for a mixture containing 70wt% tungsten. These calculations served as a comparative tool allowing a differentiation between compositions. In addition, the calculations allowed a good estimation of the temperature evolving when the mixture is burning and reinforce the claim that tungsten does not participate in the reaction.